A cold cathode fluorescent lamp (CCFL) has terminal voltage characteristics that depending upon the immediate history and the frequency of a signal (AC signal) applied to the lamp. Until the lamp is lit, the lamp will not conduct a current with an applied terminal voltage that is less than the strike voltage. Once an electrical arc is struck inside the lamp, the terminal voltage may fall to a run voltage that is approximately ⅓ of the strike voltage over a relatively wide range of input currents, as shown in FIG. 1.
Curve 1 in FIG. 1 represents a frequency-gain relation of an inverter when the lamp has been lit, while curve 2 represents the frequency-gain relation when the lamp is not lit. Generally, the quasi-resonant frequency fs0 of curve 1 is chosen to be the operating switch frequency of the inverter, so that the inverter has a large gain G1 when the lamp is in normal operation. However, as shown in FIG. 1, if the switch frequency is fs0, the gain of the inverter of curve 2 is G2, which is far lower than its maximum gain. Accordingly, the lamp may not be ignited.
In order to overcome the above disadvantage, prior art inverters choose a quasi-resonant frequency fopen0 of curve 2 as its switch frequency when the lamp is being lit, such that the inverter has a large gain G3. After a predetermined time period, the inverter chooses the quasi-resonant frequency fs0 of curve 1 as its switch frequency. However, the predetermined time period may be not long enough, so that the lamp may not be entirely ignited. Alternatively, the predetermined time period may last too long, so that the inverter still operates under a switch frequency fopen0 even if the lamp has been lit, causing the lamp to be extinguished due to a low gain of the inverter.